#define _CRT_SECURE_NO_WARNINGS
#include <set>
#include <map>
#include <list>
#include <queue>
#include <stack>
#include <cmath>
#include <ctype.h>
#include <ctime>
#include <cstdio>
#include <vector>
#include <string>
#include <bitset>
#include <cctype>
#include <cstdlib>
#include <cstring>
#include <utility>
#include <numeric>
#include <complex>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <cassert>
#include <iostream>
#include <iterator>
#include <algorithm>
#define aLL(g) (g).begin(),(g).end()
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define F(i,j,k) fill(i[0],i[0]+j*j,k)
#define P(p) cout<<(p)<<endl;
#define EXISt(s,e) ((s).find(e)!=(s).end())
#define INF 1<<25
#define MAX 100000000
#define PI 3.14159265258979
#define pb push_back
using namespace std;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<double> vd;
typedef pair<int, int> pii;
typedef pair<long, long> pll;
typedef long long ll;
int dx[] = { 0, 1, 0, -1 };
int dy[] = { -1, 0, 1, 0 };

int sttoi(std::string str) {
	int ret;
	std::stringstream ss; ss << str;
	ss >> ret;
	return ret;
}

bool sort_greater(const pair<string, int> &a, const pair<string, int> &b) {
	return a.second > b.second;
}

void solve() {
	string s, t;
	cin >> s >> t;
	bool flag = false;//s big
	if (s.length() > t.length()){
		P(s);
		return;
	}
	else if (s.length() < t.length()){
		P(t);
		return;
	}
	else{
		for (int i = 0; i < s.length(); i++){
			if (s[i] > t[i]){
				if (s[i] == '7' && t[i] == '4'){
					flag = true;
				}
			}
			else{
				if (s[i] != '4' || t[i] != '7'){
					flag = true;
				}
			}
		}
	}
	if (flag){
		P(t);
	}
	else{
		P(s);
	}
}

int main() {
	solve();
	return 0;
}